The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge(δ-) and the other atom has a slight positive charge(δ+).
If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect.
Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character is indicated by the +I effect.
As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out, and is significant only over a short distance. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.
The inductive effect may be caused by some molecules also. Relative inductive effects have been experimentally measured with reference to hydrogen.
Inductive effects can be measured through the Hammett equation.
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